A known fuel injector 1 will be described with reference to FIGS. 1, 2a and 2b. The injector 1 comprises an injector body 3 (sometimes referred to as a nozzle holder body), an injector nozzle 5 and a movably mounted injector needle 7. The injector nozzle 5 comprises a plurality of nozzle holes (not shown) which can be selectively opened and closed by the injector needle 7 to inject fuel into a combustion chamber (not shown). A spring 9 is provided in a spring chamber 11 for biasing the injector needle 7 towards a seated position in which the nozzle holes are closed.
The fuel injector 1 further comprises an equilibrium control valve 13 for controlling the injector needle 7. The control valve 13 comprises a control valve body 15 and a control valve member 17 mounted in a control chamber 19. The control valve member 17 comprises a guide barrel 21 and a stem 22 having a smaller diameter. A conical valve 23 is formed above the stem 22 for locating in a valve seat 24 formed in the control valve body 15 to close the control valve 13. An electro-mechanical solenoid 25 is provided to actuate the control valve member 17 and enable selective opening and closing of a low pressure fuel return line 27. A sidewall of the control chamber 19 forms a valve guide 29 for cooperating with the guide barrel 21 of the control valve member 17.
A fuel supply line 31 supplies fuel from a high pressure fuel pump (not shown) to the injector nozzle 5 and the spring chamber 11. The control chamber 19 is also in fluid communication with the fuel supply line 31 via a high pressure fuel passage 33.
When the control valve 13 is closed, there is no fluid communication between the spring chamber 11 and the low pressure fuel return line 27. Accordingly, the fuel pressure in the injector nozzle 5 and the spring chamber 11 equalises and the spring 9 biases the injector needle 7 to a seated position in which the nozzle holes are closed.
Conversely, when the control valve 13 is opened, a path is formed which places the spring chamber 11 in fluid communication with the low pressure fuel return line 27 resulting in a reduction in the fuel pressure in the spring chamber 11. The fuel pressure in the injector nozzle 5 is higher than the fuel pressure in the spring chamber 11 and a pressure force applied to the injector needle 7 overcomes the bias of the spring 9. The injector needle 7 lifts from its seated position and opens the nozzle holes allowing fuel to be injected into the combustion chamber, as shown in FIG. 1.
On a solenoid common rail injector, the control valve 13 plays an important part in controlling fuel leaks. A leak results in an energy loss and this has a direct effect on CO2 emissions of a vehicle using the injector 1. In use, the fuel injector 1 will experience two forms of leaks:                a. Dynamic leaks—these are leaks which result from the opening of the control valve 13 during injection; and        b. Static leaks—these are leaks between the control valve member 17 and the valve guide 29 when the control valve 13 is closed and the fuel injector 1 is not injecting.        
Static leaks are more significant since the control valve spends more time closed than it does open. Contributing factors in static leaks include: guide clearance; guide length; increased clearance for injector and engine assembly; and increased clearance due to pressure.
The static leaks within the control valve 13 due to pressure are particularly relevant in view of the continuing trend towards higher operating pressures (for example 2200 to 3000 bar) for fuel injected into the combustion chamber. The high pressure fuel within the control chamber 19 applies radial loading which can distort the control valve body 15. Similarly, radial loading is applied to the control valve member 17 which can cause it to distort. The distortion of the control valve body 15 and/or the control valve member 17 increases the clearance within the control valve 13 which can result in an increase in static leaks.
The pressure force gradient causes distortion of the control valve body 15, as illustrated by a first plot P1 superimposed on the control valve 13 shown in FIG. 2A. The pressure force gradient acting on the stem 22 is illustrated by a second plot P2 superimposed on the control valve 13 shown in FIG. 2B. The relative deflection along the length (mm) of the control valve body 15 and the control valve member 17 under pressure is shown in a graph in FIG. 3 (an enlarged view of the control valve body 15 and the control valve member 17 is shown alongside the graph). An initial clearance C between the control valve body 15 and the control valve member 17 increases to C′ proximal the inlet of the high pressure fuel passage 33. The increased clearance caused by the working pressures in the control chamber 19 can cause higher static leaks in the control valve 13.
The present invention, at least in preferred embodiments, sets out to overcome or ameliorate at least some of the problems associated with prior art fuel injectors and control valves.